Mechanical bearings can reduce friction between surfaces of parts that move relative to one another. A rotary bearing (e.g., rolling element bearing) is a bearing that carries a load using rolling elements such as cylinders and/or spheres that roll between an inner ring and an outer ring, also called inner and outer races. The load is continuously redistributed among the rolling elements in a rotary bearing, which reduces friction, wearing, and binding.
A bearing preload is an axial load that can be applied to rolling element bearings to reduce excess motion or play between the rolling elements and the surfaces of the inner and outer races. By properly preloading a rotary bearing, the rolling elements will roll rather than slide, preventing race misalignment and premature bearing wear.